What is the difference between 'Witnessing' and 'Mindfulness' from the context of meditation?

Maybe see the Quora thread on Osho's "witness": https://www.quora.com/What-is-witness-meditation-as-described-by-Osho

I can only tell from my personal experience: I've read books where "witnessing" was used synonymously with perceiving with mindfullness (sati) and without follow-up thoughts (https://en.wikipedia.org/wiki/Conceptual_proliferation)

Your sunset example seems to be a mix of different elements: The basic perception process goes from the object (external) to the eye sense organ to perception to sign (nimitta), but then stopping there (if you are right that there is no mental proliferation).

The feeling of oneness seems to be a separate, second perception from your "internal sense organ". However, it's unclear if the source of this feeling is directly the eye-consciousness, if it is mediated by the perception of "beauty", or if it's the result of a separate associative process based on the memories of what Osho told you about oneness.

You may also have a look at oneness and it's background: https://en.wikipedia.org/wiki/Nondualism However, attaining advaita usually takes years of meditation practice. Perception of beauty is said (by Sangharakshita somewhere) to provide a glimpse of it.

Mindfulness is situational awareness of the body and its context, which occurs effortlessly at all times. Precisely where that awareness occurs, I have no idea. Awareness isn't the best term, as there is no such thing as awareness, but I don't have another convenient word, at this time. Regardless, when it fully opens up, you are safe there. True mindfulness is the pinnacle of what Buddha taught. Knowing your body and its context leads to the end of suffering. This is why Thich Nhat Hanh called it the safe island of mindfulness. This is not a mindfulness that is performed by cognizant effort, however, when introduced to the idea of being mindful, one is lavished with a range of various mindfulness-based techniques, which may or may not be helpful.

Therefore, if we look at this in progressive terms, we might be able to find an answer to your question. In the beginning, as one approaches the practice of being mindful, one must exert themselves. This very exertion gives mindfulness a definition which makes it stand out from other things - it becomes a routine task, a behaviour that one must perform.

This is the preamble phase where a lot of discernment takes place, or what you might call witnessing. In this preliminary phase, the mind is going through a preparation, from which sensory preoccupations begin to diminish leading to disenchantment.

During that preparation, many questions will arise about the nature of mindfulness, mostly what it actually is, and are you doing it right. These, too, need to become part of what is discerned (or witnessed). Witnessing isn't the best term as it suggests one must push aside thoughts, feelings & sensations so that the witnessing can take place. However, these discernable things must remain intimately close, closer than close. The quality of attention that is used to discern passing phenomena (thoughts, feelings & sensations) is crucial to the whole practice.

In Zen terms, this is called 'sweeping' (popular in the Caodong school) and one sweeps with such persistence and regularity that the great empty sky suddenly opens up and all distinctions dissolve. Why is that? Because the discerning mind becomes disenchanted by what it has become caught inside of and seeks freedom as if all by itself.

Over in the Theravada tradition, in the Anguttara Nikaya and Buddhagosa's Path Of Purification, Page 684 a simile of a crow was used to illustrate the meaning of tathāgatako, which I colloquially translate as 'gone, mate!'. The crow symbolizes the disenchantment of the mind brought about by unrelenting mindfulness practice, and how it searches for another place of rest, which in Buddhist parlance is called Nirvana. It is here where one truly understands mindfulness, but not from the perspective of doing, hence it no longer becomes a task. One might say it is now the ordinary part of daily life.

The 7 Stages of Purification - Page 107

sati ("mindfulness") is one of the most commonly misunderstood teachings. It's been distorted by psychotherapy agendas, and even experienced Buddhist teachers have wildly divergent understandings.

If you want the perspective from the original suttas: There's an implicit object of sati, and that's the Dharma, the Buddha's teaching that leads to nirvana. One always remembers to apply the Dharma. All the time. The fourth frame of sati, seeing Dharma as Dharma, is commonly misunderstood to be seeing thoughts (the dhamma that the mind/mano cognizes/viññāna). But the primary role of seeing Dharma as Dharma, is living each moment in accordance with the Buddha's Dharma, and the dhamma-thoughts cognized by the mind are subservient to that primary objective.

More detail here: http://notesonthedhamma.blogspot.com/2022/12/two-ways-in-which-sati-mindfulness-is.html

the other answers are already good so I'll just add this quote from wiki regarding witness:

"When form is the object of observation or drshyam, then the eye is the observer or drk; when the eye is the object of observation, then the mind is the observer; when the pulsations of the mind are the objects of observation, then Sakshi or the Witnessing-Self is the real observer; and it is always the observer, and, being self-luminous, can never be the object of observation. When the notion and the attachment that one is the physical body is dissolved, and the Supreme Self is realized, wherever one goes, there one experiences Samadhi. "

of course I don't see how the eye can be the object of observation but that's just me...i'm not a god